This invention is concerned with sewage treatment involving aerobic digesters, and specifically the invention relates to a method of operating an activated sludge process in such a way as to minimize capital requirements, minimize aeration requirements and minimize coliform count in solids effluent.
In sewage treatment plants, aerobic digestion as a means of stabilizing sludge has been common practice for decades. During the process of digestion, nitrification of the sludge tends to occur when sufficient air is provided, normally via submerged aeration bubblers. However, if nitrification is allowed to continue unchecked, this will lead to a pH drop in the system. A low pH (e.g. about 5) will retard microbial activity, and efficiency of the process will be lost. To recover this lost alkalinity, plant operators have in the past added alkaline agents, specifically lime, to the digester. Also, it is known that on the liquid side of the system, e.g. in aeration basins upstream of clarifiers, aeration air may be shut off for a period of time to enable denitrification of the sludge to occur. This can be used to recover up to about 50% of the lost alkalinity. The abrupt shutoff of air to the microbes, which have become very active during aeration, causes the microbes to break down nitrates in the sludge, utilizing the oxygen from the nitrates and releasing nitrogen gas. Acidity in the solution is reduced in this way, and on the liquid side it has been common to cycle the air on and off to maintain pH in a desired range.
It has also been suggested to incorporate an anoxic operating period into the aerobic digestion cycle. See, for example:
Matzuda, A., T. Ide, and S. Fujii (1988), "Behavior of nitrogen and phosphorus during batch aerobic digestion of waste activated sludge--continuous aeration and intermittent aeration by control of DO," Water Res., vol. 22, 1495-1501;
Peddie, C. C., D. S. Mavinic, and C. J. Jenkins (1990), "Use of ORP for Monitoring and Control of Aerobic Sludge Digestion," Journal of Environmental Engineering, vol. 116, 461-471;
Hao, O. J. and M. H. Kim (1990), "Continuous Pre-Anoxic and Aerobic Digestion of Waste Activated Sludge," Journal of Environmental Engineering, vol. 116, 863-879; and
Hao, O. J., M. H. Kim, and I. A. Al-Ghusain (1991), "Alternating aerobic and anoxic digestion of waste activated sludge," Journal of Chemical Technology and Biotechnology, vol. 52, 457-472. As pointed out in that literature, aerobic/anoxic operation has the advantages of recovering lost alkalinity and it can reduce overall oxygen requirements in the aerobic digestion process.
A recent paper by Glen T. Daigger presented at the 70.sup.th Annual Conference & Exposition of the Water Environment Federation in Chicago, Ill. in October 1997 reviews aerobic/anoxic operation as an improvement to conventional aerobic digestion processes. As reviewed in that paper, one problem in operating conventional aerobic digestion systems is the depletion of alkalinity during aeration, caused by nitrification of released ammonia-nitrogen, which results in a decrease in pH, reducing biological reaction rates. Daigger points out that in the absence of nitrification, the overall digester reaction is as follows: EQU C.sub.5 H.sub.7 O.sub.2 N+5 O.sub.2 .fwdarw.4 CO.sub.2 +NH.sub.4 HCO.sub.3( 1)
where C.sub.5 H.sub.7 O.sub.2 N represents the typical composition of biomass. Alkalinity is produced in the form of ammonium bicarbonate, which at neutral pH will ionize to produce ammonium ions and bicarbonate ions.
With nitrification which tends to occur in the aerobic digester, the ammonia-nitrogen is converted to nitrate-nitrogen: EQU NH.sub.4.sup.+ +2 O.sub.2 .fwdarw.NO.sub.3.sup.- +2 H.sup.+ +H.sub.2 O(2)
Thus, two moles of acidity are produced, because one mole of ammonia is consumed and one mole of nitric acid is produced. Thus, the overall reaction (combining the two above equations) becomes: EQU C.sub.5 H.sub.7 O.sub.2 N+7 O.sub.2 .fwdarw.5 CO.sub.2 +3 H.sub.2 O+HNO.sub.3 (3)
This shows that process oxygen requirements are increased with the occurrence of nitrification, as indicated by an increase from five moles of oxygen per mole of biomass to seven moles of oxygen per mole of biomass, comparing the first and third equations above. The acidity produced by the nitric acid will consume some of the alkalinity produced in the first equation.
As further explained by Daigger, if an anoxic phase is included in the aerobic digestion cycle, the reaction is as follows: EQU C.sub.5 H.sub.7 O.sub.2 N+4 NO.sub.3.sup.31 +H.sub.2 O.fwdarw.NH.sub.4.sup.+ +5 HCO.sub.3.sup.- +2 N.sub.2 (4)
No free oxygen is consumed during this reaction, and alkalinity is produced, which can then be used in nitrifying the ammonia-nitrogen released from the biomass destruction. If the aerobic and anoxic periods can be controlled such that all of the nitrate-nitrogen formed during the aerobic phase is reduced in the anoxic phase, then the overall reaction becomes: EQU C.sub.5 H.sub.7 O.sub.2 N+5.75 O.sub.2 .fwdarw.5 CO.sub.2 +2 N.sub.2 +4 H.sub.2 O (5)
Equations (3) and (5) above demonstrate that the aerobic/anoxic stages introduced in aerobic digestion systems can reduce process oxygen requirements. As pointed out in the Daigger paper (the source of the above equations), equations (3) and (5) show reduction from seven moles of O.sub.2 per mole of biomass to 5.75 moles of O.sub.2 per mole of biomass, a 17% reduction. Moreover, alkalinity is not consumed in the overall reaction. Alkalinity from the destruction of biomass (equation (1)) and resulting from denitrification is that required to provide the alkalinity needed for nitrification (equation (2)). Thus, the pH of the digester, if a controlled aerobic/anoxic process is maintained, should remain near neutral.
The Daigger paper, and tests supporting the paper, suggested establishing the anoxic stage in the digester itself, by cycling aeration on and off (e.g. off for 6 to 8 hours each day).
Prior conventional practice in activated sludge digestion was to draw settled sludge from a main activated sludge process or from clarifier basin(s) and deliver this sludge directly to the digester(s), although it has been common to recycle some of the solids from the clarifier to the aeration basin to maintain a desired solids content in the aeration basin. In more recent years aerobic digester systems have been modified to bring a thicker sludge to the digester, in order to meet requirements for longer retention times without increasing tank volumes. Gravity thickeners, belt thickeners, drum thickeners, centrifuges and even recycle loops have been used, with the purpose of reducing tankage capital costs to achieve target retention times. Enviroquip's prior art Pre-thickened Aerobic Digestion (PAD) system is an example of such a system which pre-thickens sludge prior to aerobic digestion, reducing capital costs in tankage requirements. That system incorporates a premix/thickener/digester recycle loop to progressively thicken wasted sludge.
Thicker sludges delivered to the digester have created several problems, including problems of oxygen transfer, mixing and mechanical equipment. Moreover, pre-thickened systems have not been operated in a manner to achieve the benefits of the invention described herein; nor have aerobic/anoxic cycling been applied in the efficient way which forms a part of the process of the invention.
The recent enactment of stricter EPA sludge quality laws, and regulations defining compliance with sludge standards such as Class A sludge or Class B sludge, have driven efforts to improve the aerobic digestion process.
It is among the objects of this invention to improve the aerobic digestion process with a system which reduces tankage capital costs, maintains pH balance without addition of alkaline agents, reduces overall aeration requirements by as much as 17%, and enables consistent compliance with Class B sludge regulations.